Thin Film Led Comprising a Current-Dispersing Structure

ABSTRACT

A thin-film LED comprising an active layer ( 7 ) made of a nitride compound semiconductor, which emits electromagnetic radiation ( 19 ) in a main radiation direction ( 15 ). A current expansion layer ( 9 ) is disposed downstream of the active layer ( 7 ) in the main radiation direction ( 15 ) and is made of a first nitride compound semiconductor material. The radiation emitted in the main radiation direction ( 15 ) is coupled out through a main area ( 14 ), and a first contact layer ( 11, 12, 13 ) is arranged on the main area ( 14 ). The transverse conductivity of the current expansion layer ( 9 ) is increased by formation of a two-dimensional electron gas or hole gas. The two-dimensional electron gas or hole gas is advantageously formed by embedding at least one layer ( 10 ) made of a second nitride compound semiconductor material in the current expansion layer ( 9 ).

The invention relates to a thin-film LED according to the preamble ofpatent claim 1.

This patent application claims the priority of German patent application10 2004 003 986.0, the disclosure content of which is herebyincorporated by reference.

A known method for producing optoelectronic components, in particularfor producing light emitting diodes on the basis of nitride compoundsemiconductors, is based on so-called thin-film technology. In thismethod, a functional semiconductor layer sequence, which in particularcomprises a radiation-emitting active layer, is firstly grownepitaxially on a growth substrate, then a new carrier is applied to thesurface of the semiconductor layer sequence opposite to the growthsubstrate, and the growth substrate is subsequently separated off. Sincethe growth substrates used for nitride compound semiconductors, examplesof said growth substrates being SiC, sapphire or GaN, are comparativelyexpensive, this method affords the advantage, in particular, that thegrowth substrate is reusable. A growth substrate made of sapphire can bestripped away from a semiconductor layer sequence made of a nitridecompound semiconductor for example by means of a laser lift-off methodknown from WO 98/14986.

A thin-film LED is distinguished in particular by the followingcharacteristic features:

-   -   a reflective layer (mirror layer) which reflects at least part        of the electromagnetic radiation generated in the epitaxial        layer sequence back into the latter is applied or formed at a        main area of a radiation-generating epitaxial layer sequence        that faces a carrier;    -   the epitaxial layer sequence has a thickness in the region of 20        μm or less, in particular in the region of approximately 6 μm;        and    -   the epitaxial layer sequence contains at least one semiconductor        layer with at least one area which has an intermixing structure        which ideally leads to an approximately ergodic distribution of        the light in the epitaxial layer sequence, that is to say that        it has an as far as possible ergodically stochastic scattering        behavior.

A basic principle of a thin-film LED is described for example in I.Schnitzer et al., Appl. Phys. Lett. 63 (16), Oct. 18, 1993, 2174-2176,the disclosure content of which is in this respect hereby incorporatedby reference.

Electrical contact is made with thin-film LEDs generally by means of twoelectrical contact layers, for example by means of a p-type contactlayer on the rear side of the carrier and an n-type contact layer on theside of the semiconductor layer sequence that is remote from thecarrier. In general, the side of the thin-film LED that is remote fromthe carrier is provided for coupling out radiation, so that a contactlayer that is non-transparent to the emitted radiation can only beapplied to a partial region of the surface of the semiconductor layersequence. For this reason, often only a comparatively small centralregion of the chip surface is provided with a contact area (bondingpad).

In conventional light-emitting diode chips having an edge length of lessthan 300 μm, in general a comparatively homogeneous current distributionin the semiconductor chip can already be achieved by means of a bondingpad arranged centrally on the chip surface.

In the case of large-area semiconductor chips having an edge length ofapproximately 1 mm, by way of example, this type of contact-making may,however, disadvantageously lead to an inhomogeneous current feed intothe semiconductor chip, which leads to an increased forward voltage andto a lower quantum efficiency in the active zone. This effect occurs inparticular in the case of semiconductor materials which have a lowtransverse conductivity, in particular in the case of nitride compoundsemiconductors. The maximum current density occurs in a central regionof the semiconductor chip in this case. However, the radiation emittedin said central region of the semiconductor chip is at least partlyemitted toward the non-transparent bonding pad and thus at least partlyabsorbed.

In order to improve the current expansion, it is known for example toapply a thin semitransparent metalization layer, for example Pt or NiAu,over the whole area of the chip surface of a p-type semiconductormaterial. In this case, however, a non-negligible part of the emittedradiation, for example approximately 50%, is absorbed in thesemitransparent layer. Furthermore, such contact layers are not readilysuitable for making contact with n-doped nitride compoundsemiconductors.

In order to improve the coupling-in of current in the case of InGaAlPLEDs, it is known from DE 199 47 030 A1 to use a relatively thick,transparent current expansion layer provided with a laterally patternedelectrical contact layer. In this case, the current is impressed througha central bonding pad and also through a plurality of contact websconnected to the bonding pad on the chip surface. This type ofcontact-making cannot readily be applied to large-area light-emittingdiode chips which contain a semiconductor material having a lowtransverse conductivity, in particular nitride compound semiconductors,since the density of the non-transparent contact webs on the chipsurface would have to be increased in such a way that a large part ofthe emitted radiation would be absorbed in the contact layer. Acomparatively thick current expansion layer furthermore leads to anincreased voltage drop and takes up a long growth time duringproduction. Furthermore, strains may occur in a comparatively thickcurrent expansion layer and cracks may possibly be induced by saidstrains.

The invention is based on the object of specifying a thin-film LEDcomprising an improved current expansion structure which isdistinguished in particular by a comparatively homogeneous currentdistribution over the chip area in conjunction with comparatively littleshading of the chip surface by contact layer material.

This object is achieved by means of a thin-film LED having the featuresof patent claim 1. The dependent claims relate to advantageousrefinements and developments of the invention.

In a thin-film LED comprising an active layer, which emitselectromagnetic radiation in a main radiation direction, a currentexpansion layer, which is disposed downstream of the active layer in themain radiation direction and is made of a first nitride compoundsemiconductor material, a main area, through which the radiation emittedin the main radiation direction is coupled out, and a first contactlayer arranged on the main area, according to the invention thetransverse conductivity of the current expansion layer is increased byformation of a two-dimensional electron or hole gas.

The increased transverse conductivity of the current expansion layerleads to a homogeneous current feed into the active layer and therebyincreases the efficiency of the thin-film LED.

In order to form a two-dimensional electron or hole gas in the currentexpansion layer, at least one layer made of a second nitride compoundsemiconductor material having a larger electronic band gap than thefirst nitride compound semiconductor material is preferably embedded inthe current expansion layer.

The first nitride compound semiconductor material and the second nitridecompound semiconductor material advantageously each have the compositionIn_(x)Al_(y)Ga_(1-x-y)N where 0≦x≦1, 0≦y≦1 and x+y≦1, the composition ofthe second nitride compound semiconductor material differing from thecomposition of the first nitride compound semiconductor material in sucha way that the electronic band gap of the second nitride compoundsemiconductor material is larger than that of the first nitride compoundsemiconductor material. In this case, the respective material need notnecessarily have a mathematically exact composition according to theabove formula. Rather, it may have one or a plurality of dopants andalso additional constituents which essentially do not change thephysical properties of the material. For the sake of simplicity,however, the above formula includes only the essential constituents ofthe crystal lattice (Al, Ga, In, N) even though these may be replaced inpart by small quantities of further substances.

Regions having a particularly high transverse conductivity form at theinterfaces between the at least one layer made of the second nitridecompound semiconductor material and the current expansion layer made ofthe first nitride compound semiconductor material. The increasedtransverse conductivity of these regions can be explained in the bandmodel such that, at the interfaces between the first nitride compoundsemiconductor material and the second nitride compound semiconductormaterial, a bending of the band edges of the conduction band and of thevalence band occurs in each case and leads to the formation of apotential well in which a two-dimensional electron or hole gas having aparticularly high transverse conductivity occurs.

In one preferred embodiment of a thin-film LED according to theinvention, a plurality of layers made of the second nitride compoundsemiconductor material are embedded in the current expansion layer. Inthis way, a multiplicity of interfaces between the first nitridecompound semiconductor material and the second nitride compoundsemiconductor material are advantageously formed at each of which, onaccount of the band bending, a potential well forms in which atwo-dimensional electron or hole gas having a high transverseconductivity occurs. The transverse conductivity of the entire currentexpansion layer is thereby increased further in comparison with acurrent expansion layer having only one embedded layer having a largerelectronic band gap than the first nitride compound semiconductormaterial. The number of layers made of the second nitride compoundsemiconductor material is preferably between 1 and 5 inclusive.

The thickness of the at least one layer made of the second nitridecompound semiconductor material is approximately 10 nm to 100 nm, by wayof example.

The first nitride compound semiconductor material, from which thecurrent expansion layer is formed, is preferably GaN. The second nitridecompound semiconductor material is for example Al_(x)Ga_(1-x)N where0<x≦1, in which case 0.1≦x≦0.2 preferably holds true.

The at least one layer made of the second nitride compound semiconductormaterial preferably has a doping, the dopant concentration being higherin the regions adjoining the current expansion layer than in a centralregion of the layer. The increased dopant concentration in the regionsof the second nitride compound semiconductor material which adjoin thecurrent expansion layer has the advantage that an increased number offree charge carriers are present in the regions in which the transverseconductivity is increased by the formation of a two-dimensional electronor hole gas. The transverse conductivity and the current expansion areimproved further as a result.

The first and second nitride compound semiconductor materials are ineach case n-doped, by way of example. In this case, a two-dimensionalelectron gas forms at the interfaces between the first and secondnitride compound semiconductor materials. As an alternative, it is alsopossible for both the first and the second nitride compoundsemiconductor material to be p-doped in each case. In contrast to thecase mentioned previously, here a two-dimensional hole gas rather than atwo-dimensional electron gas forms at the interface between the firstand second nitride compound semiconductor materials. A furtheradvantageous variant of the invention provides for embedding a very thinn-doped layer made of the second nitride compound semiconductor materialin a current expansion layer made of a p-doped first nitride compoundsemiconductor material. In this case, a two-dimensional electron gas canbe generated in a p-doped first nitride compound semiconductor materialas well.

The active layer of the thin-film LED comprises for exampleIn_(x)Al_(y)Ga_(1-x-y)N where 0≦x≦1, 0≦y≦1 and x+y≦1. The active layermay be formed for example as a heterostructure, double heterostructureor as a quantum well structure. In this case, the designation quantumwell structure encompasses any structure in which charge carriersexperience a quantization of their energy states as a result ofconfinement. In particular, the designation quantum well structure doesnot comprise any indication about the dimensionality of thequantization. It thus encompasses, inter alia, quantum wells, quantumwires and quantum dots and any combination of these structures.

In one embodiment of the thin-film LED, at least one edge length of themain area provided for coupling out radiation is 400 μm or more,particularly preferably 800 μm or more. In particular, provision mayeven be made of an edge length of 1 mm or more, it being possible forthe main area to have, in particular, a square form. As a result of theincrease in the transverse conductivity of the current expansion layer,it is possible, even in the case of large-area thin-film LEDs, to obtaina comparatively homogeneous current distribution in the active layerwhich could not readily be realized otherwise with a conventionalcurrent expansion layer made of a nitride compound semiconductormaterial.

The first contact layer of the thin-film LED, which is arranged on themain area provided for coupling out radiation, preferably contains ametal or a metal alloy. The first contact layer is preferably a Ti—Pt—Aulayer sequence comprising, proceeding from the adjoining nitridecompound semiconductor layer, by way of example, a Ti layer having athickness of approximately 50 nm, a Pt layer having a thickness ofapproximately 50 nm and an Au layer having a thickness of approximately2 μm. A Ti—Pt—Au layer sequence is advantageously insensitive toelectromigration which might otherwise occur, for example in the case ofa first contact layer containing aluminum. Therefore, the first contactlayer is preferably free of aluminum.

The first contact layer advantageously has a lateral structurecomprising a contact area (bonding pad) and a plurality of contact webs.In one preferred embodiment, the contact area is surrounded by at leastone frame-type contact web, the frame-type contact web being connectedto the contact area by means of at least one further contact web. The atleast one frame-type contact web may have for example a square,rectangular or circular form.

On account of the increased transverse conductivity of the currentexpansion layer, in the case of a thin-film LED according to theinvention, it is advantageously necessary for only a comparatively smallproportion of the main area to be covered by the contact layer.Advantageously, only less than 15%, particularly preferably less than10%, of the total area of the main area is covered by the first contactlayer. The good transverse conductivity of the current expansion layerfurthermore has the advantage that even a comparatively coarsepatterning of the contact layer suffices to produce a comparativelyhomogeneous current density distribution in the active layer in thethin-film LED. By way of example, the contact area is advantageouslysurrounded by 1, 2 or 3 frame-type contact webs. Finer patterning of thecontact layer, in particular the use of a larger number of frame-typecontact webs, is not necessary in order to increase the efficiency ofthe thin-film LED on account of the high transverse conductivity of thecurrent expansion layer. The outlay for the patterning of the firstcontact layer is therefore advantageously low.

A further preferred embodiment contains a second contact layer, which,as seen from the active layer, is situated opposite to the first contactlayer. The second contact layer has a cutout in a region opposite thecontact area. The second contact layer is thus patterned in such a waythat, as seen from the active layer, a region not covered by the secondcontact layer is situated opposite the contact area which together withat least one contact web forms the first contact layer. This has theadvantage that the current density is reduced in a region of the activelayer which lies below the contact area. This is advantageous inparticular if the first contact layer comprises a non-transparent metal,because otherwise at least part of the radiation generated below thecontact area would be absorbed in the contact area. The efficiency ofthe thin-film LED is advantageously increased in this way.

The second contact layer is preferably a layer that is reflective forthe emitted radiation. This is advantageous in particular when thethin-film LED is connected to a carrier by means of a connecting layer,for example a solder layer, at an area opposite to the main area. Inthis case, the radiation emitted in the direction of the carrier isreflected back from the reflective contact layer toward the main areaand the absorption of radiation in the carrier and/or the connectinglayer is reduced in this way.

The invention is particularly advantageous for thin-film LEDs which areoperated with a current intensity of 300 mA or more, since aninhomogeneous current distribution that would have a maximum in acentral region of the light-emitting diode chip would be observed atsuch high operating current intensities in conventional thin-film LEDs.

The invention is explained in more detail below on the basis ofexemplary embodiments in connection with FIGS. 1 to 7.

In the Figures:

FIG. 1A shows a schematic illustration of a cross section through athin-film LED in accordance with a first exemplary embodiment of theinvention,

FIG. 1B shows a schematic illustration of a plan view of the thin-filmLED in accordance with the first exemplary embodiment of the invention,

FIG. 2A shows a schematic illustration of a cross section through athin-film LED in accordance with a second exemplary embodiment of theinvention,

FIG. 2B shows a schematic illustration of a plan view of the thin-filmLED in accordance with the second exemplary embodiment of the invention,

FIGS. 3A and 3B show a schematic illustration of the electronic bandstructure of an n-doped semiconductor layer in which an n-doped layerhaving a larger electronic band gap made of a second semiconductormaterial is embedded,

FIG. 4 shows a schematic illustration of the profile of the dopantconcentration of the semiconductor layers illustrated in FIG. 2,

FIG. 5 shows a schematic illustration of the band model of asemiconductor layer in which a plurality of semiconductor layers made ofa second semiconductor material having a larger electronic band gap areembedded,

FIG. 6 shows a schematic illustration of the profile of the valence bandedge of a p-doped semiconductor layer in which a p-doped semiconductorlayer made of a second semiconductor material having a larger electronicband gap is embedded, and

FIGS. 7A and 7B show a schematic illustration of the band model of ap-doped semiconductor layer in which an n-doped semiconductor layer madeof a second semiconductor material having a larger electronic band gapis embedded.

Identical or identically acting elements are provided with the samereference symbols in the Figures.

The thin-film LED in accordance with a first exemplary embodiment of theinvention as illustrated in FIG. 1A along a cross section of the lineI-II from the plan view shown in FIG. 1B contains an epitaxial layersequence 16 comprising an active layer 7. The active layer 7 is formedfor example as a heterostructure, double heterostructure or as a quantumwell structure. The active layer 7 emits electromagnetic radiation 19,for example in the ultraviolet, blue or green spectral range, in a mainradiation direction 15. The active layer 7 is contained for examplebetween at least one p-doped semiconductor layer 6 and at least onen-doped semiconductor layer 8. The electromagnetic radiation 19 emittedin the main radiation direction 15 by the active layer 7 is coupled outfrom the thin-film LED through a main area 14.

On an opposite side to the main area 14, the epitaxial layer sequence 16is fixed on a carrier 2 by means of a connecting layer 3, for example asolder layer. The rear side of the carrier is provided with an electrode1, by way of example.

In order to make electrical contact with the epitaxial layer sequence 16of the thin-film LED, a first contact layer 11, 12, 13 is provided onthe main area 14 of the thin-film LED. A current expansion layer 9containing a first nitride compound semiconductor material, preferablyGaN, is contained between the active layer 7 and the first contact layer11, 12, 13. Embedded in the current expansion layer 9 made of the firstnitride compound semiconductor material is at least one layer 10 made ofa second nitride compound semiconductor material, preferably made ofAlGaN, that is to say that the current expansion layer 9 is a multilayerlayer comprising for example two GaN partial layers 9 a, 9 b separatedfrom one another by an embedded AlGaN layer 10. The AlGaN layer 10preferably has the composition Al_(x)Ga_(1-x)N where 0.1≦x≦0.2.

As will be explained in greater detail below, the transverseconductivity of the current expansion layer 9 is improved by thesemiconductor layer 10 embedded in the current expansion layer 9. Thelayer 10 made of the second nitride compound semiconductor material andembedded in the current expansion layer 9 preferably has a thickness offrom 10 nm to 100 nm inclusive.

The first contact layer 11, 12, 13 preferably comprises a Ti—Pt—Au layersequence (not illustrated) comprising, proceeding from the adjoiningcurrent expansion layer 10, by way of example, a Ti layer having athickness of approximately 50 nm, a Pt layer having a thickness ofapproximately 50 nm and an Au layer having a thickness of approximately2 μm. In order to avoid electromigration, the first contact layer 11,12, 13 preferably contains no aluminum. The lateral structure of thefirst contact layer 11, 12, 13 arranged on the main area 14 of thethin-film LED is illustrated in the plan view illustrated in FIG. 1B.The first contact layer comprises a contact area 11 arranged in acentral region of the main area 14. The first contact layer furthermorecomprises a plurality of contact webs 12 which run from the contact area11 in the radial direction toward the edge of the thin-film LED. Saidcontact webs 12 are connected to one another at least in part by meansof further frame-type contact webs 13 enclosing the contact area 11.

The frame-type contact webs 13 may be embodied as squares or rectanglesnested in one another, as illustrated. As an alternative, by way ofexample, circular frames or frames in the form of regular polygons wouldalso be possible, the frame-type contact webs 13 preferably beingarranged concentrically, that is to say having a common mid-point, atwhich the contact area 11 is preferably arranged. The number offrame-type contact webs is preferably 1, 2 or 3. The first contact layercomprising the contact area 11 and the contact webs 12, 13 is preferablyformed from a metal, in particular from aluminum.

A second contact layer 5, which preferably produces an ohmic contact tothe adjoining semiconductor layer 6, adjoins that side of thesemiconductor layer sequence 16 of the thin-film LED which faces thecarrier 2. The second contact layer 5 preferably contains a metal suchas, for example, aluminum, silver or gold. In the case of a p-dopedsemiconductor layer 6 adjoining the second contact layer 5, silver, inparticular, is a suitable material for the second contact layer 5 sincesilver produces a good ohmic contact to p-doped nitride compoundsemiconductors.

The second contact layer 5 is preferably a layer that reflects theemitted radiation. This has the advantage that electromagnetic radiationemitted in the direction of the carrier 2 by the active layer 7 isreflected at least in part toward the main area 14 and is coupled outthere from the thin-film LED. Absorption losses that might occur forexample within the carrier 2 or in the connecting layer 3 are reduced inthis way.

The second contact layer 5 preferably has a cutout 18 in a regionsituated opposite the contact area 11 of the first contact layer. Thesize and the form of the cutout 18 preferably essentially match the sizeand the form of the contact area 11. Since no ohmic contact between thesecond contact layer 5 and the adjoining semiconductor layer 6 arises inthe region of the cutout 18, there is a reduction of the current flowbetween the first contact layer 11, 12, 13 on the main area 14 and theelectrode 1 on the rear side of the carrier 2 through the region of thecutout 18. The current flow through a region of the active layer 7 whichis arranged between the first contact area 11 and the cutout 18 in thesecond contact layer 5 is advantageously reduced in this way. Thegeneration of radiation in this region of the active layer 7 isconsequently reduced, as a result of which the absorption of radiationwithin the non-transparent contact area 11 is advantageously at leastpartly reduced.

A barrier layer 4 is preferably contained between the second contactlayer 5 and the connecting layer 3. The barrier layer 4 contains TiWN,by way of example. The barrier layer 4 prevents, in particular, adiffusion of material of the connecting layer 3, which is a solder layerfor example, into the second contact layer which might impair inparticular the reflection of the second contact layer 5 functioning as amirror layer.

The second exemplary embodiment of a thin-film LED according to theinvention as illustrated schematically in a cross section in FIG. 2A andin a plan view in FIG. 2B differs from the first exemplary embodiment ofthe invention as illustrated in FIG. 1 first of all by virtue of thefact that, instead of a single layer, three layers 10 a, 10 b, 10 c madeof the second nitride compound semiconductor material are embedded inthe current expansion layer 9, that is to say that the current expansionlayer 9 is a multilayer layer comprising for example four GaN partiallayers 9 a, 9 b, 9 c, 9 d separated from one another by three embeddedAlGaN layers 10 a, 10 b, 10 c. As an alternative, still further layersmade of the second nitride compound semiconductor material could also beembedded in the current expansion layer 9 made of the first nitridecompound semiconductor material.

A preferred number of the embedded layers is between 1 and 5. Theplurality of layers 10 a, 10 b, 10 c each have a thickness of from 10 nmto 100 nm and need not necessarily be arranged periodically. By way ofexample, the layers 10 a, 10 b, 10 c have different thicknesses and/orare at different distances from one another.

By virtue of the plurality of embedded layers 10 a, 10 b, 10 c made ofthe second nitride compound semiconductor material, the transverseconductivity of the current expansion layer 9 is advantageouslyincreased further in comparison with the embodiment with an individualembedded layer as illustrated in FIG. 1. By way of example, three layers10 a, 10 b, 10 c embedded in the current expansion layer 9 result in theproduction of six interfaces between the first nitride compoundsemiconductor material and the second nitride compound semiconductormaterial having the larger electronic band gap. At each of saidinterfaces a respective potential well for electrons forms within whichthe electrons have a particularly high mobility.

One advantage of increasing the transverse conductivity of the currentexpansion layer is that by embedding the layers 10 a, 10 b, 10 c made ofthe second nitride compound semiconductor material in the currentexpansion layer 9, the transverse conductivity of the current expansionlayer 9 is increased in such a way that the number of contact webs, thedistance between the contact webs and the chip area covered by thecontact webs 12, 13 and the contact area 11 can be reduced withoutthereby significantly impairing the current expansion within thethin-film LED.

As can be discerned in the plan view illustrated in FIG. 2B, the secondexemplary embodiment of the invention differs from the first exemplaryembodiment of the invention furthermore by virtue of the fact that thefirst contact layer on the main area 14 comprises only two frame-typecontact webs 13 instead of three frame-type contact webs 13. Byincreasing the transverse conductivity of the current expansion layer 9,it is thus possible to simplify the structure of the first contact layer11, 12, 13, thereby reducing the production outlay and reducing theabsorption of radiation within the contact layer 11, 12, 13.

The increase in the transverse conductivity by the formation of atwo-dimensional electron or hole gas is explained in more detail belowwith reference to FIGS. 3 to 7. FIG. 3A schematically illustrates theelectronic band structure in the band model of a semiconductor layermade of a nitride compound semiconductor material, for example n-dopedGaN, in which is embedded a semiconductor layer made of a second nitridecompound semiconductor material having a larger electronic band gap, forexample n-doped AlGaN. FIG. 3A schematically shows the profile of theconduction band 20, and of the valence band 21, and also the Fermi level22 of GaN and the Fermi level 23 of AlGaN, the interaction between thesemiconductor materials not having been taken into account. On accountof the larger electronic band gap of AlGaN in comparison with GaN, thedistance between the conduction band 20 and the valence band 21 isgreater in the AlGaN layer embedded in the GaN layer than in theadjoining GaN layer.

FIG. 3B shows the profile of the conduction band edge 21 taking accountof the interaction of the two semiconductor materials. Since the Fermilevels 22, 23 match one another, a band bending occurs in the regions ofthe GaN layer adjoining the AlGaN layer such that in these regions arespective potential well 25 for electrons forms in which the electronshave such a high mobility that a two-dimensional electron gas forms inthis region.

FIG. 4 schematically illustrates the profile of the dopant concentrationδ as a function of a spatial coordinate z, which runs perpendicular tothe current expansion layer, that is to say parallel to the mainradiating direction, for a preferred embodiment of the current expansionlayer. In this exemplary embodiment, an AlGaN layer is embedded in acurrent expansion layer made of GaN, both the GaN layer and the AlGaNlayer being n-doped in each case. The AlGaN layer has a higher dopantconcentration in the regions 24 adjoining the GaN layer than in itsinner portion (so-called doping spikes). The number of free electronswhich have a high mobility in the potential wells 25 illustrated in FIG.3B is therefore increased further and the transverse conductivity isconsequently improved further.

Instead of embedding only a single layer made of a second nitridecompound semiconductor material in a current expansion layer made of afirst nitride compound semiconductor material, as was illustrated on thebasis of the band model in FIG. 3, it is also possible to insert aplurality of layers made of the second nitride compound semiconductormaterial, as was explained above for example on the basis of the secondexemplary embodiment of the invention. FIG. 5 illustrates for this casethe profile of the conduction band 20 and of the valence band 21 withouttaking account of the interaction between the semiconductor materials,for example GaN and AlGaN. Taking account of the interaction, the bandbending explained in connection with FIG. 3B and the correspondingformation of potential wells (not illustrated) occur in each case ateach of the interfaces between the semiconductor materials.

The current expansion layer and the semiconductor layer made of thesecond nitride compound semiconductor material that is embedded thereinneed not necessarily be n-doped in each case. As an alternative, bothmay also be p-doped, by way of example. FIG. 6 schematically shows theprofile of the valence band edge 21 for the case of a p-doped AlGaNlayer embedded in a p-doped GaN layer. In this case, at each of theinterfaces a band bending occurs, which respectively represent potentialwells 26 for holes. In this way, a two-dimensional hole gas may in eachcase be generated in the interface regions.

In a further preferred embodiment of the invention, in order to generatea two-dimensional electron gas within a p-doped current expansion layer,for example p-GaN, an n-doped layer, for example n-AlGaN, which has alarger electronic band gap than the p-doped layer, is embedded in thecurrent expansion layer. The undisturbed band model of this embodimentis schematically illustrated in FIG. 7A and the band model takingaccount of the interaction of the semiconductor layers is schematicallyillustrated in FIG. 7B. Analogously to the example illustrated in FIG.3B, wherein both the GaN layer and the embedded AlGaN layer are n-dopedin each case, a respective potential well 25 for electrons forms, inthis case, too, at the interface between the p-doped GaN and the n-dopedAlGaN on account of the band bending at the semiconductor-semiconductorinterfaces, by virtue of the formation of a two-dimensional electrongas, having an increased transverse conductivity.

The invention is not restricted by the description on the basis of theexemplary embodiments. Rather, the invention encompasses any new featureand also any combination of features, which in particular comprises anycombination of features in the patent claims, even if this feature orthis combination is itself not explicitly specified in the patent claimsor exemplary embodiments.

1. A thin-film LED comprising: an active layer, which emitselectromagnetic radiation in a main radiation direction; a currentexpansion layer, which is disposed downstream of the active layer in themain radiation direction and is made of a first nitride compoundsemiconductor material; a main area, through which the radiation emittedin the main radiation direction is coupled out; and a first contactlayer arranged on the main area, wherein the transverse conductivity ofthe current expansion layer is increased by formation of atwo-dimensional electron gas or hole gas.
 2. The thin-film LED asclaimed in claim 1, wherein in order to form a two-dimensional electronor hole gas in the current expansion layer, at least one layer made of asecond nitride compound semiconductor material having a largerelectronic band gap than the first nitride compound semiconductormaterial is embedded in the current expansion layer.
 3. The thin-filmLED as claimed in claim 2, wherein a plurality of layers made of thesecond nitride compound semiconductor material are embedded in thecurrent expansion layer.
 4. The thin-film LED as claimed in claim 2,wherein the number of layers made of the second nitride compoundsemiconductor material is between 1 and 5 inclusive.
 5. The thin-filmLED as claimed in claim 2, wherein the at least one layer made of thesecond nitride compound semiconductor material has a thickness of 10 nmto 100 nm.
 6. The thin-film LED as claimed in claim 2, wherein the firstnitride compound semiconductor material is GaN.
 7. The thin-film LED asclaimed in claim 2, wherein the second nitride compound semiconductormaterial is Al_(x)Ga_(1-x)N where 0.1≦x≦0.2.
 8. The thin-film LED asclaimed in claim 2, wherein the at least one layer made of the secondnitride compound semiconductor material has a doping, the dopantconcentration being higher in the regions adjoining the currentexpansion layer than in a central region of the layer.
 9. The thin-filmLED as claimed in claim 2, wherein the first and second nitride compoundsemiconductor materials are n-doped.
 10. The thin-film LED as claimed inclaim 2, wherein the first nitride compound semiconductor material isp-doped and the second nitride compound semiconductor material isn-doped.
 11. The thin-film LED as claimed in claim 1, wherein the activelayer (7) includes In_(x)Al_(y)Ga_(1-x-y)N where 0≦x≦1, 0≦y≦1 and x+y≦1.12. The thin-film LED as claimed in claim 1, wherein at least one edgelength of the main area (14) is 400 μm or more.
 13. The thin-film LED asclaimed in claim 12, wherein at least one edge length of the main areais 800 μm or more.
 14. The thin-film LED as claimed in claim 1, whereinoperation of the thin-film LED with a current intensity of 300 mA ormore is provided.
 15. The thin-film LED as claimed in claim 1, whereinthe first contact layer (11, 12, 13) comprises no aluminum.
 16. Thethin-film LED as claimed in claim 1, wherein less than 15% of the totalarea of the main area is covered by the first contact layer.
 17. Thethin-film LED as claimed in claim 1, wherein the first contact layer hasa lateral structure comprising a contact area and a plurality of contactwebs.
 18. The thin-film LED as claimed in claim 17, wherein the contactarea is surrounded by at least one frame-type contact web, theframe-type contact web being connected to the contact area by means ofat least one further contact web.
 19. The thin-film LED as claimed inclaim 18, wherein the frame-type contact web has a square, rectangularor circular form.
 20. The thin-film LED as claimed in claim 18, whereinthe number of frame-type contact webs is one, two or three.
 21. Thethin-film LED as claimed in claim 1, wherein a second contact layer,which reflects the emitted radiation, is provided on a side of theactive layer opposite to the first contact layer, the first contactlayer having a contact area and the second contact layer having a cutoutin a region opposite the contact area.